A diabatic correlation diagram procedure, first applied (Rose, J. P.; Kellman, M. E., J. Chem. Phys. 1996, 105, 7348) to H2O, is used to classify energy and intensity patterns in the highly excited bending spectrum of C2H2, modeled by an effective spectroscopic fitting Hamiltonian. Analysis with polyad phase spheres accounts for the observed patterns in terms of classical phase space structure and bifurcations of the low-energy normal modes.